The present invention relates to an operational amplifier made up of MOSs (metal oxide semiconductors).
There has been known an operational amplifier of the type described in "Analog MOS Integrated Circuit," pp 39-40 published by IEEE PRESS, 1980, edited by Paul R. Oray et al. The two circuit arrangements shown in FIGS. 1 and 2 are examples of operational amplifiers having a bias circuit. In FIG. 1, a differential amplifier 10, a source follower circuit 20 and an output circuit 30 cooperate to form an operational amplifier. A bias circuit 40 provides bias voltage for the operational amplifier. The differential amplifier 10 is comprised of a pair of N-channel MOS transistors 13 and 14 which are respectively connected at the gates to input terminals 11 and 12 for differential voltage signals, a pair of P-channel MOS transistors 15 and 16 which constitute a current mirror circuit and serve as a load circuit for the transistor pair 13 and 14, and an N-channel MOS transistor 17. The differential amplifier 10 amplifies the difference between the input signals applied to the gates thereof by way of the input terminals 11 and 12, and produces a differentially amplified signal. The source follower circuit 20 is comprised of an N-channel MOS transistor 21 as a drive transistor and an N-channel MOS transistor 22 as a load transistor. Applied to the gate of the MOS transistor 21 is the output signal from the operational amplifier 10. Applied to the gate of the MOS transistor 22 is a bias voltage from the bias circuit 40. The source follower circuit 20 is for shifting the level of output voltage from the differential amplifier 10. The shifted voltage is input to the output circuit 30 comprised of a P-channel MOS transistor 31 and an N-channel MOS transistor. The gate of the MOS transistor 32 is tied with the output of the source follower circuit 20. The gate of the MOS transistor 31 is connected to the output voltage of the source follower circuit 20. The output circuit 30 produces, at an output terminal 33 connected to a junction between the MOS transistors 31 and 32, a voltage signal corresponding to the differential input voltages applied thereto.
The bias circuit 40 is for applying bias voltage to the gates of the transistors 17 and 22 in the differential amplifier 10 and to the source follower circuit 20. The bias circuit 40 is comprised of three MOS transistors 41-43 connected in series between a high potential source V.sub.DD and a low potential source V.sub.SS. The MOS transistor 41 is of the P-channel type, while the MOS transistors 42 and 43 are of the N-channel type. Those transistors 41-43 are used as impedance elements for voltage dividing.
The operational amplifier thus arranged is essentially accompanied by a variation in the power voltage between the high potential source V.sub.DD and the low potential source V.sub.SS, and by a variation of the threshold voltage of each MOS transistor. Those variations create an offset in the output voltage derived from the output terminal of the operational amplifier. To prevent such an offset, those transistors are geometrically designed so that the gate - voltage of the MOS transistors 15, 16 and 31 is equal to that of the MOS transistors 41 in the bias circuit 40, so that the gate - voltage of the MOS transistor 21 is equal to that of the MOS transistor 42 in the bias circuit 40, and so that the gate - voltage of the MOS transistors 17, 22 and 32 is equal to that of the MOS transistor 43 in the bias circuit 40. This approach succeeds in eliminating the offset due to the variation in the power voltage and threshold voltage.
Nevertheless, the variation in the power voltage still directly appears in the gate - voltage of those transistors, resulting in a great variaton in the consumption current. For example, a 10% change in the power voltage between V.sub.DD and V.sub.SS comes to approximately a 20% change in the consumption current. The end result is that the approximately 10% change of the mutual conductance "gm" greatly influences the gain and band width of the operational amplifier. This narrows the ability of the power voltage to provide an operational amplifier with a good characteristic.
The operational amplifier shown in FIG. 2 uses a constant voltage source 50 in place of the bias circuit 40 used in FIG. 1. Usually the constant voltage source 50 which is incorporated in the operational amplifier is provided separately from the operational amplifier. The constant voltage source 50 may be a zener diode incorporated into the operational amplifier. The MOS transistor 22 applied with a bias voltage from the constant voltage source 50 serves as a constant current source, and always feeds a constant current to the MOS transistor 21. As a result, the gate - voltage of the MOS transistor 21 is constant. The gate - source paths of the MOS transistors 31, 21 and 32 are inserted in series between the high potential source V.sub.DD and the low potential source V.sub.SS, as shown. Therefore, the voltages across these gate - source paths are equal to the power voltage between the V.sub.DD and V.sub.SS. As was just mentioned, the gate - voltage of the MOS transistor 21 is constant. When the power voltage between the V.sub.DD and V.sub.SS varies, the gate - voltage of each of MOS transistors 31 and 32 then varies. Therefore, when the power voltage increases, the gate - voltage of each of MOS transistors 31 and 32 becomes larger, in order to set the output voltage from the output terminal 33 at a midpoint between the power voltage between the V.sub.DD and V.sub.SS. Since the gate - voltage of the MOS transistor 31 is equal to the source - drain voltage of the MOS transistor 16 in the differential amplifier 10, any increase of the power voltage leads to an increase of the source - drain voltage of the MOS transistor 16. Under this condition, when the source - drain voltage between the MOS transistors 15 and 16 is put out of balance, an offset will appear in the output voltage from the signal output terminal 33. The gate - voltage of the transistors 31 and 32 varies with the variation of the power voltage. Therefore, the range of the power voltage of the operational amplifier which ensures a good characteristic is narrowed, as in the FIG. 1 circuit.